Our A330s (all ETOPS) do not have this message (just checked the FCOMs).
Sounds like a nicety installed on the newer models, or perhaps a company option. Definately not an ETOPS requirement. Like your FCOM states "Crew awareness"...
Sounds similar to the FOB BELOW 3T memo on (some) A320s. Company option that one I think.
Strange that you state it is an ECAM Caution though, are you certain it's not just a memo? Doesn't seem to fit the caution definition "An operating procedure, technique, etc. that may result in damage to equipment if not followed".